Description of the Prior Art
The conventional method of producing chlorinated methanes involves the reaction of methane with chlorine gas. For each substitution of a chlorine atom into the methane molecule, one molecule of hydrogen chloride is produced. Thus, double the amount of chlorine is consumed compared with the quantity incorporated into the desired chlorinated hydrocarbon. In other words, the maximum chlorine efficiency is 50 percent. Since the cost of chlorine is a major factor in the cost of producing chlorinated methanes, any inefficiency in its use is a severe handicap.
Alternative chlorination methods have been tried over the years with varying success. The object of these methods has been to produce chlorinated methanes without the coproduction of hydrogen chloride. For example, by starting with methyl alcohol (methanol) and hydrogen chloride, methyl chloride can be produced. This produce is useful by itself, or in turn it can be reacted with chlorine to give methylene chloride and hydrogen chloride. Since the latter can be recycled to the methanol reaction step, the net production of hydrogen chloride is zero.
While the above scheme, which starts with methanol, is used commercially, it nevertheless has certain drawbacks. To begin with, methanol is more expensive than methane from which it is produced. Furthermore, only methyl chloride or methylene chloride can be made in balanced reactions. If the more highly chlorinated methane products, namely, chloroform or carbon tetrachloride are desired, excess hydrogen chloride must be disposed of.
In order to circumvent the shortcomings of existing technology, numerous attempts have been made to oxychlorinate methane. Methods, for example, employing oxyhalogenation and related technology are described in U.S. Pat. Nos. 3,470,260, 2,334,033, 2,498,546, 3,173,962, 3,345,422, 4,000,205, 4,020,117, 4,284,833, 4,386,28, and 4,446,249.
Although oxychlorination appears in theory to offer advantages, there are many technical difficulties with the process. For example, at sufficiently high temperatures which are required for chlorination, some of the methane begins to burn with the air. Such combustion may lead to the formation of hot spots in the catalyst bed thereby complicating the problem of temperature control. With overheating, the catalyst may gradually lost its efficiency. Also, whatever hydrocarbon is burned reduces the yield of product. Finally, there is the ever present danger of explosions should, for one reason or another, the supply of hydrogen chloride to the reactor be interrupted.
It is therefore an object of the present invention to provide a method for the chlorination of methane that overcomes the disadvantages of the conventional methods.
It is also an object to provide a method of the kind described with includes endothermic and exothermic reactions, namely substitution chlorination and dissociation, that are carried out in tandem such that the overall energy requirements can be closely balanced.
These and other objects, features and advantages of the invention will be apparent from the following description and the accompanying drawing in which: